DSL (Digital Subscriber Line) provides Internet users with a technology that provides higher bandwidth and takes advantage of the installed base of copper twisted-pair telephone wire that exists between the telephone company central offices (COs) and local residential customers. ADSL (Asymmetric Digital Subscriber Line) is a form of DSL technology that transmits more information in one direction than the other. ADSL separates the available frequencies in a wire on the assumption that most residential users download much more information than they upload.
In DSL systems, analog echo cancellation is used to prevent saturation of the receiver circuitry by the transmit signal. A cancellation device in a DSL modem models the transmit echo path transfer function to attenuate the unwanted transmit signal and noise that appears in the receiver path. The line impedance, and therefore the transmit echo, varies greatly for different loop topologies and numbers of bridged-taps. Therefore, a single cancellation device cannot achieve the same echo cancellation levels for all possible loop scenarios.